OPHTHALMOSCOPE
OPTIC NEURITIS
affects the brain or the nerves supplying
the eye muscles, such as
stroke
,
a
brain
tumour
,
encephalitis
,
or
multiple sclerosis
.
ophthalmoscope
An instrument used
to examine the inside of the
eye
.
ophthalmoscopy
A noninvasive proce-
dure in which an ophthalmologist (a
doctor specializing in eye disorders)
uses an
ophthalmoscope
to examine the
inside of the
eye
. The ophthalmoscope
is used first to direct a beam of light
into the eye and then to examine the
light-sensitive
retina
; the retinal blood
vessels; the head of the
optic nerve
; and
the jelly-like
vitreous humour
.
OPHTHALMOSCOPY
op iate Any drug derived from, or chemi-
cally similar to,
opium.
opioid A type of
analgesic drug
(pain-
killer) used to treat moderate to severe
pain. Opioids, also knows as narcotic
drugs, may be abused for their euphoric
effects; abuse may cause
tolerance
(the
need for greater amounts of a drug to
get the same effect), and physical and
psychological
drug dependence.
Com-
monly used opioids include
codeine
,
diamorphine
,
morphine
,
and
pethidine
.
opium
A substance obtained from the
unripe seed pods of the poppy plant
papaver
somniferum.
Opium
has
an
analgesic effect and may also cause
sleepiness and euphoria. Opium and its
derivatives, such as
codeine
and
diamor-
phine
,
are known as
opioid
.
opportunistic infection
Infection by
organisms that rarely have serious or
widespread effects in people of normal
health, but which can cause serious ill-
ness or widespread infection in a person
whose
immune system
is impaired. In
most patients with
AiDS
, death is due
to opportunistic infections, especially
pneumocystis pneumonia
. Many fungal
infections, such as
candidiasis
, and some
viral infections, such as
herpes simplex
,
are opportunistic infections. Treatment
is with appropriate antimicrobial drugs.
oppositional defiant disorder
A type
of behavioural disorder that usually
appears in childhood or early adoles-
cence. Typically, a child shows hostile,
argumentative behaviour that includes
loss of temper, defiance of rules, and
swearing. To some extent such behaviour
is common in adolescence, but when
law-breaking or violence occur the con-
dition is deemed to be pathological,
optic atrophy A shrinkage or wasting
of the
optic nerve
fibres due to disease
or injury to the optic nerve, resulting in
partial or complete loss of vision. Optic
atrophy may occur without prior signs
of nerve disease, such as inflammation,
optic disc The area on the
retina
where
nerve fibres from the eyeball join the
optic nerve.
The optic disc is also known
as the blind spot due to its lack of light-
sensitive cells.
op tician A person who fits and sells
glasses
or
contact lenses.
An ophthalmic
optician, or optometrist, also examines
the eyes to test for
myopia, presbyopia,
hy'permetropia,
or
astigmatism.
People
with suspected eye disorders are referred
to a specialist called an ophthalmologist.
(See also
ophthalmology'; optometry'.)
optic nerve The 2nd
cranial nerve;
the
nerve of
vision.
The 2 optic nerves each
consist of about 1
million nerve fibres
that transmit impulses from the
retina
to the
brain
. The optic nerves converge
behind the eyes, where fibres from the
inner halves of the retina cross over.
Nerve fibres from the right halves of
both retinas go to right side of the
occipital lobes in the brain; those from
the left halves go to the left side.
Disorders of the optic nerve include
optic neuritis
and
papilloedema
. The latter
is caused by pressure on the nerve from
disease in the
orbit
or a
brain tumour
.
optic neuritis
Inflammation of the
optic
nerve
, often causing sudden loss of part
of the visual field. Attacks are sometimes
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